In the consumer audio sector, for example, the power for the loudspeakers is transmitted by way of correspondingly voluminous copper wires. In addition to high costs, these cables also act as transmitting antennas of high-frequency interference, if switching amplifiers, so-called D amplifiers, are used. Within a greater progressive concept, a D amplifier that has a digital data stream supplied to it via light guides is built into the housing of the loudspeaker. The D amplifier of such a loudspeaker is supplied by means of a power network connector, and must also fulfill strict standards with regard to electromagnetic system perturbation by the environment.
In the case of such known solutions, conversion of the input data stream into an analog signal is always provided, which signal is then amplified and the amplified signal in turn is subsequently converted back to a digital signal. In this connection, however, a quality loss due to the repeated conversion cannot be avoided.
In addition, the degree of effectiveness of non-switching analog amplifiers, at approximately 50%, is relatively low Currently, switching amplifiers, i.e. those that contain power electronics, already reach degrees of over 90%. In the case of the D amplifier, the analog voltage applied to the input is pulse-width modulated (PWM) at a fixed frequency, the switching frequency of the D amplifier. This PWM voltage is amplified in the PWM end stage of the D amplifier by means of alternately switching transistors having a high degree of effectiveness on and off. The filter size of a D amplifier is determined by the lower auditory threshold (15 Hz). The degree of effectiveness is significantly influenced by the switching frequency. The switching frequency must amount to a multiple of the upper auditory threshold (20 Hz).
The spectrum of the amplified PWM voltage naturally has a great switching-frequency component and great harmonic components, which now have to be completely filtered out of the voltage again, accordingly, in order to obtain a high-power analog voltage that is as undistorted as possible at the load. In this connection, it is necessary to avoid pulse inaccuracies by means of corresponding feedback.
D amplifiers having an analog input and passive filters are known from the state of the art. The phase rotation of a band-pass filter and the complex loudspeaker load very quickly set narrow limits for the strong negative feedback that is desirable for low distortions, because of instabilities.
Another problem in known D amplifiers of this type also consists in the fact that direct voltage power supplies that essentially derive a sine-shaped current from a network, so-called power factor correction circuits (abbreviated as PFC hereinafter) deliver wave-shaped direct voltage that has a significant ripple component, i.e. ripple. In this connection, the ripple has a frequency that corresponds to 2n times the frequency of the network, whereby n stands for the number of phases of the network connected to a full-path rectifier circuit. However, a noticeable ripple of the voltage supply makes extensively distortion-free amplification of the input signal difficult. In order to smooth out ripple, DSDC stages are usual, but they require a relatively great effort/expenditure of switching technology.
U.S. Pat. No. 5,559,467 discloses an amplifier circuit using a D amplifier, the analog output signal of which is fed back to a “Noise and Ripple Shaping Chip.” This “Noise and Ripple Shaping Chip” furthermore has a digital input signal and an analog signal from the voltage supply supplied to it.
An overview of PFC (“power factor corrected”) rectifiers is offered by the article “A Topology Survey of Single-Stage Power Factor Corrector with a Boost Type Input-Current-Shaper,” Qiao, C. et al., APEC 2000, 15th Annual IEEE Applied Power Electronics Conference and Exposition, New Orleans, LA, Feb. 6–10, 2000, Annual Applied Power Electronics Conference, New York, N.Y.: IEEE, US, Vol. 1 of 2, Conf. 15, Page 460–467, XP001036175, ISBN: 0 7803 5865 1.
Another overview of PFC power supplies, also in connection with D amplifiers, is offered by the article “A Class D Amplifier Using MOSFETs with Reduced Minority Carrier Lifetime,” J. Hancock, Journal of the Audio Engineering Society, Audio Engineering Society, New York, US, Vol. 39, No. 9, Page 650–662, XP000226144, ISSN: 0004–7554.
In U.S. Pat. No. 6,107,876, an amplifier circuit for a digital input signal is described, in which an analog signal is fed back.